In the field of an electrophotographic photoreceptor which constitutes an image-forming apparatus such as a copier or a printer employing an electrophotographic method, there was proposed a method of adding a conductive filler having a low powder resistance in the surface layer of the photoreceptor as a resistance component in order to prevent generation of image faults such as memory in the photoreceptor. When the above-described conductive filler is added to the surface layer of the photoreceptor, if the powder resistance of the conductive filler is too low, the charging property of the photoreceptor will become lowered to result in producing charging failure and fog. Therefore, it is required to suitably adjust the powder resistance of the added conductive filler. As a method of adjusting the powder resistance of the conductive filler, there was done a surface treatment by using a coupling agent such as a silane coupling agent (refer to Patent document 1: JP-A No. 2009-53727).
On the other hand, in order to improve the cleaning property, there was known a method of adding a lubricating filler which gives a low friction property to the surface layer of the photoreceptor, for example, organic particles made of polytetrafluoroethylene (PTFE) (refer to Patent document 2: JP-A No. 2011-197443), or composite particles composed of inorganic particles and a fluorine resin (refer to Patent document 3: JP-A No. 2011-128546).
However, in the case of adding the lubricating filler to the surface layer, the dispersion condition of the lubricating filler in the coating liquid for forming the surface layer may be insufficient, since the lubricating filler will repel the resin which constitutes the surface layer due to the low surface energy of the lubricating filler. As a result, it is difficult to obtain a sufficient strength against scratching. There is even a case in which addition of the lubricating filler may cause a cleaning failure. When the lubricating filler is an insulator, the electric property of the surface layer may be deteriorated.
The problem of the dispersibility of the lubricating filler in the coating liquid may be resolved by using a dispersion auxiliary agent. When this dispersion auxiliary agent is an insulator, the electric property of the surface layer may be deteriorated.
There is a limitation of regulating the powder resistance of the conductive filler to the required condition by the surface treatment. Further, when it is used the conductive filler having been subjected to a surface treatment with a coupling agent, there may be produced a problem relating to the low dispersion in the coating liquid in the same manner as the lubricating filler.
As described above, the use of the conductive filler or the lubricating filler is effective as a method of obtaining a surface layer having high strength with excellent cleaning property, and a required electric property for forming an image of high quality. However, it is difficult to fully obtain the required properties that are possessed by the conductive filler or the lubricating filler.